(a) Field
The described technology relates generally to a transmission method. More particularly, the described technology relates generally to a transmission method in a WLAN.
(b) Description of the Related Art
A frequency band used in the WLAN is an unlicensed band, and other wireless devices, for example Bluetooth devices, besides WLAN devices can use the same frequency band. Therefore, the WLAN devices use a carrier sense multiple access (CSMA) protocol to avoid collisions with the other WLAN devices or the other wireless devices. In the CSMA protocol, the WLAN device detects energy on a channel and transmits the signal only when the channel is not being used. In this case, the WLAN device occupies the channel by transmitting a request to send (RTS) frame or a clear to send (CTS) frame. The other devices set a network allocation vector (NAV) based on a duration field of the RTS frame or CTS frame and do not perform the contention for a channel access during the NAV duration.
A basic service set (BSS) operating on the same channel as a device's BSS and within (either partly or wholly) its basic service area (BSA) may exist in the WLAN. This BSS is called an overlapping basic service set (OBSS). A current WLAN can a wide bandwidth using a primary channel together with a second channel. For example, the IEEE standard 802.11ac may use bandwidths such as 20 MHz, 40 MHz, 80 MHz, and 160 MHz through a secondary channel of 20 MHz, a secondary channel of 40 MHz, and a secondary channel of 80 MHz as well as the primary channel of 20 MHz.
If a device within the BSS sets a NAV by a PPDU [PLCP (physical layer convergence procedure) protocol data unit] from the OBSS, the device cannot use an empty secondary channel due to the NAV set by the OBSS even if the PPDU of the OBSS does not use a part of the secondary channels. Accordingly, there is a problem that the channels cannot be efficiently used.